Single Molecule Transistor - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 18, NO. 1, JANUARY 1, 2018

High-Sensitivity Sensor Using C60-Single Molecule Transistor Abdelghaffar Nasri, Aïmen Boubaker, Bilel Hafsi, Wassim Khaldi, and Adel Kalboussi

Abstract— In this paper, we use the Extended Huckel Theory coupled with a non-equilibrium green function to calculate the transport proprieties of molecular single electron transistor (MSET). We present the effect of electrode materials on the transport properties in an MSET using a Fullerene (C60 ) molecule. The operation and performance of this MSET as a gas sensor are studied for the first time using a Fullerene as sensing material. The MSET sensors exhibited a change in the electric characteristic when the sensors were exposed to different gases. The favorable NH3 response characteristics of the MSET sensors were observed from the change in the drain-source current as a function of voltage, when the sensors were exposed to various NH3 concentrations ranging from 40 to 200 ppb. A satisfactory selectivity and remarkable high sensitivity have been found. The present results make our MSET as a very powerful candidate for use in gas sensing/measuring instrument, which allows the detection of toxic gas at low concentration. Index Terms— Gas sensor, selectivity, sensitivity, single molecule transistor, organic semiconductors, fullerene, quantum molecular devices.

I. I NTRODUCTION

M

ATERIAL sciences world is changing rapidly and especially organic semiconductors which have created critical and great part of the interest in the academic group and scientific research owing to their minimal cost and flexibility properties [1]. A great part of effort in research activities has focused on new types of functionalities and properties of molecular electronics devices to save power consumption and miniaturization problems. In additions, opening the way for building nano-dispositive for several applications like organic light-emitting diodes (OLED) [2], solar cells [3], molecular sensors [4], molecular memory devices [5], gate logic circuit [6], [7] single molecule transistor [8]. The fullerene is a family of carbon compounds, It contains 2(10+N) carbon atoms. We are interested in the Fullerene (C60 ) having 60 carbon atoms forming carbonaceous spheres where the carbon atoms are arranged in semi-regular polyhedral distributed over the sphere as 12 pentagons and Manuscript received August 15, 2017; revised October 26, 2017; accepted October 31, 2017. Date of publication November 3, 2017; date of current version December 7, 2017. The associate editor coordinating the review of this paper and approving it for publication was Prof. Danilo Demarchi. (Corresponding author: Abdelghaffar Nasri.) A. Nasri , A. Boubaker, W. Khaldi, and A. Kalboussi are with the Microelectronics and Instrumentation Laboratory, Faculty of Science of Monastir, University of Monastir, Monastir 5019, Tunisia (e-mail: [email protected]). B. Hafsi is with the CNRS, Centrale Lille, ISEN, University of Valenciennes and Hainaut-Cambresis, 59300 Famars, France, and also with the UMR 8520-IEMN, Lille University of Science and Technology, 59000 Villeneuve-d’Ascq, France. Digital Object Identifier 10.1109/JSEN.2017.2769803

Fig. 1.

Fullerene (C60 ) molecule.

20 hexagons Fig.1. The most striking property of the C60 molecule is its high symmetry. There are 120 symmetry operations, like rotations around an axis or reflections in a plane, which map the molecule onto itself. Hence, C60 is the most symmetric molecule. in addition, it has a small diameter equal to 7Å [10]. Organic Fullerenes (C60 ) compounds, the third carbon form, became important molecules in scientific research. Owing to their practical unusual and useful properties. In recent years, this material is one in all the foremost encouraging materials full-grown dramatically [9], [10]. The Fullerene is widely used for construction of electrochemical biosensors. It was used as sensor materials to detect volatile polar substances, such as gaseous NH3 [11]–[13]. In 1974, Aviramet et al. proposed the concept of using a single molecule to produce a rectifying molecular diode [14]. The first single electron transistor using C60 molecule was made in 2000 [15]. There have been multiple theoretical and experimental works that focused on the transport properties of Au-C60-Au junction [16]–[19]. A single electron transistor (SET) is a three-terminal nanoelectronic device, where, unlike a conventional field effect transistor, a quantum dot (QD) or molecule works as the channel region. Utilizing the well-understood properties of atoms and molecules, nanotechnology proposes the construction of novel molecular devices possessing extraordinary properties. The SET may be a key component of nanotechnology research that is able of giving small size and low power consumption. In this respect, we test the MSET based on C60 as a nanogas-sensor for the first time. The gas detection is very important in commercial applications; it recognizes the presence of different gases in an environment according to our needs such as for instance, those dealing with food quality measurements. The sensor property depends on changes in the resistivity because of molecules adsorbed on the detected components. Some analytical models have been directed toward extending these studies to sensor devices [20], such as organic–inorganic heterojunction diodes as gas sensors [21], metal oxide gas

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NASRI et al.: HIGH-SENSITIVITY SENSOR USING C60 -SINGLE MOLECULE TRANSISTOR

sensor [22], carbon nanotubes- graphene [15] phosphorene and graphene [24], organic thin film transistors as gas sensor [25] microelectromechanical systems [26] and HEMT sensors [27]. The investigation of SET as a gas sensor has been studied earlier, which is a new way to detect toxic gases at low concentration [28]–[30] In this study, we have used the NEGF formalism coupled with the EHT, which is implemented in Atomistix ToolKit (ATK) package [31]–[34]. In this paper, the I-V characteristics of a single molecule transistor based on C60 molecule are studied. We concentrated on the electrodes impact on the electron transport and the Coulomb staircase state. In the second part, we test the C60 -MSET as nano-gas sensor with different gases molecules (NH3 , NO2 , CO and H2 O); an acceptable sensitivity has been found. In our study, we have considered the change in the I-V curves as the detection mechanism to calculate the properties of our gas sensor. II. C OMPUTATIONAL M ODELS AND M ETHODS We have used the ATK simulator to study the electrical characteristics of MSET. In particular, we study the electron transport through an organic molecule between gold or platinum surfaces at a temperature equal to 300 K. The electrode Poisson solver calculations was set out with boundary conditions as periodic and the Brillouin zone integration equal to 1 × 10 × 100 k-point sampling. For computing electrostatic potentials, we have utilized a real space grid, with mesh cutoff energy equal to 75 Hartree. The source and drain electrodes are extracted along (111) direction of bulk. The large supercell dimension take the perpendicular direction of electron transport there is no interaction or less between the C60 molecule and their mirror. In addition, we used 22 Å supercell in the y-direction. As illustrated in [23], we need the integration of the transmission function T (E) to calculate the current with arbitrary channel material. Non Equilibrium Greens Function (NEGF) formalism provides a well-defined way for calculating T (E). It has been reported by Supriyo Datta et al. [35] that the transmission coefficient, T (E), for an electron with energy E (going from the source to the drain) can be defined as follows: T (E, V ) = Tr [τ R (E, V )Gc(E, V )τ L (E, V )G + c c(E, V )]

(1)

This expression describes the level broadening due to the coupling between left (L) and right (R) electrodes and the central scattering region (S). The sum represents the retarded self-energies associated with this coupling. The method proposed in [36], uses the obtained transmission coefficient, T(E), the conductance could be calculated by the Landauer formula: −∂ f 2e2 d E T (E) . (2) G= h ∂E In addition, the zero-bias current through the device at voltage V could be calculated: V V 2e +V /2 T (E, VG ) f (E − )− f (E + ) d E (3) I= h −V /2 2 2

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Fig. 2. (a) Single molecule transistor with gold electrode. (b) Single molecule transistor with platinum electrodes.

Where f α (E) =

1 1+ex p

E−E F KBT

is the Fermi-Dirac distribution

function T is the temperature, VG is the gate voltage and kB = 8.610−5 eV/K is Boltzmann’s constant. Under non-equilibrium conditions, the I–VDS characteristic could be calculated from the voltage-dependent transmission, T (E, V D S ), as I =

2e h

+∞ −∞

T (E, V D S ) [ f S (E) − f D (E)] d E

(4)

Where the fS,D are the electrochemical potential in the drain and source. The presence of gas molecule leads to a change the transmission coefficient. As a result, after gas exposure, the current also changed [37]. Our device configuration consists of a C60 molecule setup in a SET geometry as shown in the Fig.2b and 2b with Gold and Platinum source-drain electrodes, respectively. We use a metal back-gate with 3.8 Å of dielectric material HfO2 . The C60 molecule used as a sensor material. In addition to the absence of any direct contact to the molecule, finite conductivity in such a SET occurs through the process of sequential tunneling. The Coulomb repulsion between the charge carriers maintain this segregation, giving rise to the isolation of the molecule between source and drain. To keep the barriers’ similarity, we create a distance equals to 2.8 Å between the molecule and the electrodes. In addition, we have taken into consideration the existence of van der Waals repulsion between the molecule and the dielectric surface to make our device nearer to reality [26], [38]. We fixed the molecule above the dielectric with on a distance equals to 1.2 Å. Many researchers have studied the influence of the geometry and position of electrodes on the sensitivity and selectivity of sensors [39]–[43]. The increase in the sensitivity follows an increase in the spacing between the electrodes. The geometry of the electrode and the distance between the electrodes are interdependent. We have optimized the MSET configuration to achieve at the same time; the presence of Coulomb blocking phenomena on the one hand and to obtain good performance as a gas sensor on the other hand. The distance and flat-flat electrode geometry used allow the most stability geometry to obtain a good characteristic for MSET with Coulomb blockade phenomena.

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Fig. 3. The I_VDS curves of C60 SET in the bias range from 0.0 to 2.0 V with two different electrodes. Current flows from the left electrode to the right electrode, inset: present the electrode effects on conductance curve.

Fig. 4. Transmission T(E) as a function of electron energy (with respect to Fermi energy level) for devices with two electrodes (Au and Pt).

III. S IMULATION R ESULTS

current value. The Platinum based MSET has a higher current than gold MSET, suggesting that Pt based MSET is a better candidate for the low powered SET device. Here, we briefly outlined the fundamental electronic transport properties of the MSET with two different electrodes (Pt or Au). The shape and largeness of the current steps strongly depend to the electrode material. Furthermore, the particular mechanism can be interpreted by analyzing the transmission spectrum. Since the current through the molecule in MSET given by the Landauer– Büttiker formula, as indicated above, an analysis of the transmission spectra of Fig. 4 allows us to understand the I–V characteristics. The altitude of the step in the I-V curve is on the zone of the corresponding peak in the transmission spectrum appeared illustrated in Fig. 4 [31]. The electrode dependence of the conductance of C60 single molecular transistor is studied. Here, the result of the single C60 molecule bridging between Pt and Au electrodes in the SET structure is discussed. Our result is compared with the previous experimental and theoretical results of gold and platinum electrodes. In addition, Bohler et al. [44] determined that the conductance of the Au/C60 /Au junction was 0.1 G0 (G0 = 2e2/h), and Kiguchi et al. determined that the conductance of the Pt/C60 / Pt junction was 0.7 G0 in [45] and the conductance of Au/C60 /Au junction was 0.2(±0.1) in [46]. All references above [44]–[46] prove that the conductance of the Pt/C60 / Pt junction was higher than that of the Au/C60 /Au junction. We notice that the conductance has a different value of C60 molecular junction. The conductance of C60 -MSET was determined to be 0.2 G0 and 0.9 G0 with gold and platinum electrodes, respectively. This study has a good agreement with previous studies [16]–[19], [44]–[46], the conductance of the platinum MSET was higher than that of the gold MSET. These investigations revealed that the C60 molecule adsorbs on the Pt surface via strong covalent bond, while the C60 molecule weakly adsorbs on the Au surface without forming the covalent bond. The calculated transmission function profile shown in these works [16]–[19], [44]–[46] has an obvious difference in the transmission peak location with the one shown in this work.

Generally, the Coulomb blockade effect makes the electrons not free to move from source to drain. The electrons number move one-by-one. In this context, we propose to study the electrical characteristics of C60 “MSET”. In the first part, we focus on the transport properties of SET device based on Fullerene (C60 ) called “MSET”. We have used the Atomistix ToolKit package to set up Fullerene (C60 ) molecule in a SET environment. The I_V characteristic was obtained, by apply in a gate voltage of 1 V and the sourcedrain voltage is taken over a range of −2 to 2 V. In the second part, we test the MSET as gas sensor; the sensing performance of our sensor has been investigated. A. The I-V Characteristic of MSET: Electrode Effect In this section, we present the results of simulating a single C60 molecule in a SET environment with two different electrodes (Au and Pt). The I_VDS curve plotted for the negative and positive values of VDS to check the similarity in barriers. The Fig.3 shows the electrode effect on the I–V characteristic of the MSET. The finite conduction occurs through the process of sequential tunneling. By injecting a charge in the molecule, an image charge in the extremely polarizable contact surface is established. This, in turn, induces a powerful Coulomb attraction between the two closely separated charges situated at the interface of the electrode/molecule. The resulting charge that blocks the injection/ejection of one charge into/from a molecule is named “Coulomb blockade effect”. The Coulomb staircase takes place periodically with rising source-drain voltage, once this voltage is enhanced, extra electrons will be introduced within the molecule. Thus, every step shown in the I-V curve corresponds to a further electron being introduced within the molecule. The Fig.3 shows the coulomb Staircase state on the I_V curves. Generally, the applied source-drain voltage changes the electrode potential value. The energy level in the source contact (μS ) giving rise to a value equals to (qVDS ). When the bias is large enough, the molecule conducts. In addition, the change of electrode material can change the transport properties, amount energy of (qVDS) and the source-drain


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Fig. 5. The I-V response of the C60 -MSET sensor at different gases molecule (40ppb).

In addition, the distance between molecule and electrode, electrode shape, and the SET environment affect on the transport properties and transmission function profile as described in [46]–[48]. These kinds of C60 MSET could be a good candidate for future generation of Nano-electronic devices for small consumption, faster operation, and small size. Unlike inorganic SET devices, the Single Molecule Transistor has tinier size and operates at room temperature, which has a great advantage for industrial applications such as being used to measure low concentration gases.

Fig. 6. I response of the C60 -MSET sensor at different gases (40ppb), an electron donor (NH3 red, CO black), an electron acceptor (NO2 blue, H2 O green).

B. Single Molecule Transistor as Gas Sensor Based on many researches in literature, gold and platinum have a good performance gas sensor application [49]–[51]. However, the biggest disadvantage to use the platinum-MSET for gas detection, it has a higher cost than gold material. In this section, we study the MSET device described above with a gold electrode as a gas Nano sensor. In order to evaluate the selectivity of our sensor, the sensing performance of MSET is investigated. We have computed the I-V features of C60MSET gas sensor transistors at various gas insertions (NH3, NO2 , CO and H2 O) [52]–[54]. The C60 -MSET I-V characteristics change after exposure to different gases (NH3 and CO, NO2 and H2 O) as shown in Fig. 5. In fact, the NH3 and CO are electron donor and the NO2 and H2O are acceptors, tend to increase and decrease the electron density in C60 -MSET respectively. From Fig. 5, we can see that when the NH3 or CO gas are introduced, the drain-source current of the C60 -MSET sensor showed an obvious rise. However, the drain-source current could reduce after the device is reintroduced to NO2 and H2 O. In general, gas sensors based on a semiconductor are characterized by their low selectivity. Our device shows a satisfactory selectivity with respect to the semiconductor gas sensor. The current curves shown in Fig.4 display similar trend with no obvious difference when bias is smaller than 1V, it can be explained by the time response of our device. Based on the experiments studies carried out by Schedin et al. [55] for a graphene based transistor. The adjustment in resistivity changes when gas-induced has different magnitudes for various gas.

Fig. 7. Change in I-V response of the C60-MSET sensor at different gases molecule concentration, inset: I-V logarithmic representation.

In the same direction of their works, we noticed the current (resistivity) changes in our device by introducing different acceptor and donor molecule gases at a room temperature as illustrated in Fig. 6. Along these lines, our work prove that our device can work as a gas sensor. In order to understand the current change (resistivity), after exposing the MSET to the gases, we have calculated the I. As we have mentioned above that the NH3 and CO are electron donor molecules always increase the current flowing in the conducting channel. However, at low voltage, we noted a decrease of current, this effect can be explained by the existence of hydrogen or oxygen atoms which act as energy barriers [56]. To evaluate the sensitivity of our device, the ammonia sensitivity was investigated at 300 K by monitoring the percentage change in the drain current of C60 -gold device of various concentrations ranging from 0 to 200 ppb of ammonia NH3 . The concentration effect on the I-V curves is illustrated in Fig. 7. It is obvious that the current variation in response to the introduction of ammonia gas is found, and the drain current is significantly increased with the increase in ammonia concentration. On one hand, the adsorption of NH3 molecule onto the C60 can increase the carrier density, which increases the conductivity between the source and the drain electrodes.

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linear response characteristic. This certainly indicates the good sensitivity of our device for ammonia gas detection. It can detect the toxic gas at low concentrations. These qualities prove that our MSET is a good gas sensor. IV. C ONCLUSION

Fig. 8. Change in relative response vs different concentration of NH3 exposures.

We note, for a concentration equals to 160 ppb and 200 ppb, the I-V curves increase rapidly. It can be explained by the fact that the electron density rises as well. In addition, theoretical studies predicted that the NH3 interaction with carbon atoms would raise conductivity [57]. On the other hand, several studies showed that no charge transfer happens between an organic semiconductor and the NH3 [58]. The exchange of charge between the NH3 and gold contacts is also shown to be negligible [58] and the molecules stay neutral. The only explanation is that the NH3 molecules are intrinsically polarized; it can modify the metalmolecule energy barrier before and after gas exposure. Gold has a work function of about 5.1 eV while C60 has a work function equal to 4.5 eV. U is the potential drop from the metal to the C60 in the gap, and q V D S is the band bending at the end of the C60 . The gas molecules can modify both of them. We note that, the tunnel barrier for electrons Be before gas exposure is obtained as the difference between q Au and qC60 . After gas exposure, the charged NH3 molecules can modify this tunnel barrier. When NH3 concentration increases, the tunneling barrier decreases and the current conduction increases. In addition and under same condition, the Coulomb steps features will be small. The system still works as a SET. We can notice that the gas molecules adsorbed can modulate the tunneling properties. We proved that gas molecule did act as a secondary gate terminal as described in [29]. However, the saturation current of the device is proportional to the NH3 gas concentration. It is well-known that the change in the carrier transfer could lead to an increase in the drainsource current. We have used the current under voltage of 2 V as the saturation current under different concentrations to calculate the sensor relative response. The sensing relative response is defined as: Relative response (S) =

I g − I bg × 100 I bg

(5)

Where Ibg is the current before the gas exposure and Ig is the current after gas exposure. The sensing response versus the concentration is illustrated in Fig. 8. Obviously, the relative response is increased with the increase in ammonia concentration. We found an almost

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Abdelghaffar Nasri received the M.S. degree in microelectronics and instrumentation from the University of Monastir, Tunisia, in 2013, where he is currently pursuing the Ph.D. degree in microelectronics and nanostructure. His research activities concern the molecular electronic devices, such as gas sensor, gate logic, and single electron device.

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Aïmen Boubaker received the M.C. degree in electronic engineering from National School of Engineering of Sfax (ENIS), Tunisia, in 2006, and the Ph.D. degree in physics and electronics from The Lyon Institute of Nanotechnology (INL), France, in 2010. In 2006, he was with the Preparatory Institute of Nabeul, Tunisia, as an Assistant Professor. He joined the Department of Physics in the Faculty of Sciences Monastir, Tunisia, and INL. He has been working on the electrical characterization, modeling and simulation of OFETs and organic compounds with the cooperation of Institute of Electronics, Microelectronics and Nanotechnology since 2012. He is the author of seven review articles in international refereed journals and about 17 papers published in national and international conferences.

Bilel Hafsi received the Ph.D. degree in microelectronics from the Institute of Electronics, Microelectronics and Nanotechnology, Lille, France, in 2016. During his Ph.D., he was involved in electronic properties of organic devices in the frame of a partnership with the Microelectronics Laboratory, University of Monastir, Tunisia. His research activities concern the nanofabrication, characterizations and simulation of nano-devices, such as organic field effect transistors and memories, and the study of devices-based graphene and nanoparticles for bioinspired applications. From 2013 to 2016, he was a Teacher with Telecom Lille working in different capacities, but mainly teaching in the field of electronics. He is currently attached to education and research at Ecole Centrale de Lille.


Wassim Khaldi received the M.S. degree in microelectronics and instrumentation from the University of Monastir, Tunisia, in 2014, where he is currently pursuing the Ph.D. degree in microelectronics and nanostructure. His research interests include organic rectifier diode, organic thin film deposition, hybrid structure and TCAD simulation

Adel Kalboussi received the M.S. degree in semiconductor physics from the University of Monastir, Tunisia, in 1997, and the Ph.D. degree in semiconductor materials and devices from the National Institute of Applied Science of Lyon in 2001.